Rather than detracting from Beowulf's epic stature, the companions actually add to it. Oral epos is normally structured agglutinatively rather than analytically. In a hyperliterate society, one might simply analyze the inefficacy of prior attacks to show the superiority of Beowulf's strength and strategy. In oral epos, the effect is created by repitition, with the failures of previous attackers build up reedundantly to make Beowulf's final triumph more dramatic by contrast.
More importantly, heroism is not purely individual in Old English epic. To portray Beowulf as a great king requires showing him as a charismatic leader, attracting numerous followers. Although the followers fail to kill Grendel, their heroic abilities enhance the prestige of Beowulf, and his having followers even at his young age, shows in him the capacity for the kingship he will attain in a manner that single combat would not.